We describe vertical and temporal variation of water mass structure, dissolved nutrients, phytoplankton chlorophyll a(Chla), and dissolved organic matter south of the island of Puerto Rico at 67 ¿W, 17 ¿36'N with particular emphasis on variability of Caribbean Surface Water (CSW) features. CSW at the Caribbean Time Series (CaTS) station experiences yearlong continental riverine influence as evidenced by surface salinity consistently below that of Tropical Atlantic Surface Water and elevated silicate content. Salinity exhibits close correlation to average rainfall over the Orinoco River basin with a 3--4 month offset, and salinity and silicate are in turn closely correlated. Calculations using a simple mixing model and based on known end-members for salinity and silicate indicate that rainwater accounts for 0--3.1% of CSW, while river water accounts for 0.6--5.5% at CaTS, indicating that surface waters of the NE Caribbean are under the influence of riverine water throughout the year. No significant correlation was found between salinity and phytoplankton Chl a, but the depth of the Chl a maximum was statistically correlated to seasonal salinity fluctuations. This observation may account for the apparent seasonal Chl a increase in these waters deduced from remote-sensing estimates. Analysis of historical data shows that seasonal shoaling of the Chl a maximum results in a net increase in phytoplankton carbon biomass during the period of influence of the Orinoco River plume. ¿ 2001 American Geophysical Union